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Friction Problem -- String is tied to a block and a hanging bucket

  1. Nov 19, 2014 #1
    USE OF THE PHYSICS FORUMS TEMPLATE IS REQUIRED ON ALL HOMEWORK PROBLEMS. PLEASE COMPLY WITH THIS IN THE FUTURE.

    A string is tied to a 4.4 kg block and 120g hanging bucket. Students add 20g washers one at a time to the bucket. The student are unaware that the

    coefficient of static friction for the block on the table is 0.42.
    A) what is the maximum force of static friction for the block?
    Answer: 18.11N
    B) how many washers can the students add to the bucket without moving the block?
    Answer: 86
    C) the coefficient on kinetic friction is 0.34. Calculate the acceleration of the block when the final washer is added to the bucket and the objects start to move...

    How do get c)? I got 0.56m/s^2 but the answer in the book is 0.75m/s^2
     
    Last edited by a moderator: Nov 19, 2014
  2. jcsd
  3. Nov 19, 2014 #2
    Is it correct to assume that there is a pulley present somewhere?

    Chet
     
  4. Nov 19, 2014 #3
    ya there is a pulley between the bucket and the block. Basically, the block is sitting on the table and there is a pulley at the edge of the table. The bucket is hanging from the string that is on the pulley. its like the usual diagrams.

    I also did the same procedure but I am not getting that answer!
     
    Last edited: Nov 19, 2014
  5. Nov 19, 2014 #4
    Show us how you got your answer? How can we help you if we.don't know the details of what you did?

    Chet
     
  6. Nov 19, 2014 #5
    a= m2g -Fs/ m1 + m2
    a= (1.86 x 9.8) - 15/ (4.4 + 1.86)
    a= 0.52m/s^2


    I got Fs by...
    Fs = (0.34)(Fn=4.4x9.8)
    = 1.86

    I got m2 by...
    1. calculating the mass of the washers
    20g x 87 washers = 1740g
    2. adding the bucket and washers
    120g + 1740g = 1860g
    3. converted to kg
    1860g/1000g=1.86 kg
     
  7. Nov 19, 2014 #6
    Your methodology is solid. I get 0.57 m/s2, in rough agreement with your 0.56. The book shows 0.75, but they got the digits interchanged. For the kinetic friction force, I got 14.7 N.

    Chet
     
  8. Nov 19, 2014 #7
    Just a second confirmation; I get 0.58 m/s2 as well.
     
  9. Nov 19, 2014 #8
    Hi Brul. Welcome to Physics Forums.

    Chet
     
  10. Nov 19, 2014 #9
    Hello and thank you!
     
  11. Nov 19, 2014 #10
    Thanks. I rounded the kinetic force to 15 which I probably should avoid.
     
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